1. Field of the Invention
The invention relates to an axial-flow turbine, essentially consisting of an outer casing, an inner casing with a preferably integrated vane carrier and a rotor fitted with rotor blades, in which turbine the first stage is designed as a radial/axial stage, the radial vane row being supplied from a toroidal or spiral inlet flow housing.
2. Discussion of Background
The supply to axial bladings, in particular of low-pressure parts of steam turbines, can be effected by a toroidal annular space. This has the task of feeding the steam rate entering this annular space through one or more pieces of pipe to the first blade ring as uniformly as possible and with avoidance of major losses. Because of the limited number and the sometimes asymmetrical arrangement of the feed pipe branches, this is not achievable to an adequate extent. The large number of the necessary deflections of the flow, until the radial blade channel is reached, causes losses which can reach a multiple of the kinetic inflow energy in the pipe branch. For this reason, endeavors are made to minimize the mean velocities in the annular space, which lead to large dimensions of the annular space. Because non-uniform inflow to the axial blading is to be expected, a radial guide grid, which generates the spin necessary for producing power in the first rotor wheel, is therefore arranged in the radial inflow part. Such a turbine is known, for example from DE-A-No. 2,358,160.
A spiral design of the inlet flow housing, such as has been known already a long time ago in water turbines, allows an increase in the mean inflow velocity by a multiple of the values usual for toroidal inlet channels, without reaching the large losses of the latter. This is possible as a result of the fact that the flow direction which, in the inlet branch and in the spiral, is predominantly tangential in the same direction as that of the turbine rotation, can be utilized directly for producing work. The friction losses, which are also increased due to the higher velocities, are of less importance by comparison. By means of a suitable design of the cross-sections of the spiral, uniform inflow to the radial blade channel can be achieved and a radial guide grid, arranged there, will then deflect the flow only weakly and hence with low losses. Such a turbine is known, for example, from DE-A-No. 2,503,493.
To absorb forces due to different expansions during operation, the inlet flow housings are as a rule provided with reinforcing ribs or bars distributed around the periphery are provided in the radial channel upstream of the radial vane row, as can be seen in DE-A-No. 2,358,160 already quoted. It is obvious that such channels represent quite considerable flow resistances.